Datasheet ADXL213 (Analog Devices) - 9

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ADXL213 THEORY OF OPERATION PIN 8 XOUT = 80% YOUT = 50% PIN 8 TOP VIEW PIN 8 X (Not to Scale) X OUT = 50% OUT = 50% Y Y OUT = 20% OUT = 80% XOUT = 50% YOUT = 50% PIN 8 XOUT = 20% YOUT = 50% EARTH'S SURFACE
04742-0-015 Figure 21. Output Response vs. Orientation The ADXL213 is a complete dual axis acceleration measure- After being low-pass filtered, the duty cycle modulator converts ment system on a single monolithic IC. It contains a polysilicon the analog signals to duty cycle modulated outputs that can be surface-micromachined sensor and signal conditioning read by a counter. A single resistor (RSET) sets the period for a circuitry to implement an open-loop acceleration measurement complete cycle. A 0 g acceleration produces a 50% nominal duty architecture. The output signals are duty cycle modulated cycle. The acceleration can be determined by measuring the digital signals proportional to acceleration. The ADXL213 is length of the positive pulse width (t1) and the period (t2). The capable of measuring both positive and negative accelerations to nominal transfer function of the ADXL213 is ±1.2 g. The accelerometer can measure static acceleration forces such as gravity, allowing the ADXL213 to be used as a tilt Acceleration = ((t1/t2) – Zero g Bias)/Sensitivity sensor. Where in the case of the ADXL213 The sensor is a surface-micromachined polysilicon structure Zero g Bias = 50% nominal built on top of the silicon wafer. Polysilicon springs suspend the structure over the surface of the wafer and provide a resistance Sensitivity = 30%/g nominal against acceleration forces. Deflection of the structure is mea- sured using a differential capacitor that consists of independent t2 = RSET/125 MΩ fixed plates and plates attached to the moving mass. The fixed
PERFORMANCE
plates are driven by 180° out-of-phase square waves. Accelera- Rather than using additional temperature compensation tion deflects the beam and unbalances the differential capacitor, circuitry, innovative design techniques have been used to ensure resulting in an output square wave whose amplitude is propor- that high performance is built in. As a result, there is essentially tional to acceleration. Phase sensitive demodulation techniques no quantization error or nonmonotonic behavior, and are then used to rectify the signal and determine the direction temperature hysteresis is very low (typically less than 10 mg of the acceleration. over the –40°C to +85°C temperature range). The output of the demodulator is amplified and brought off- Figure 9 shows the zero g output performance of eight parts (X chip through a 32 kΩ resistor. At this point, the user can set the and Y axis) over a –40°C to +85°C temperature range. signal bandwidth of the device by adding a capacitor. This filtering improves measurement resolution and helps prevent Figure 12 demonstrates the typical sensitivity shift over aliasing. temperature for VS = 5 V. Sensitivity stability is optimized for V S = 5 V, but is still very good over the specified range; it is typically better than ±2% over temperature at VS = 3 V. Rev. A | Page 8 of 12 Document Outline REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION PERFORMANCE APPLICATIONS POWER SUPPLY DECOUPLING SETTING THE BANDWIDTH USING CX AND CY SELF TEST DESIGN TRADE-OFFS FOR SELECTING FILTER CHARACTERISTICS: THE NOISE/BW TRADE-OFF USING THE ADXL213 WITH OPERATING VOLTAGES OTHER THAN 5 V USING THE ADXL213 AS A DUAL-AXIS TILT SENSOR Dual-Axis Tilt Sensor: Converting Acceleration to Tilt PIN CONFIGURATIONS AND FUNCTIONAL DESCRIPTIONS OUTLINE DIMENSIONS ESD CAUTION ORDERING GUIDE AUTOMOTIVE PRODUCTS